The present invention relates to an all wheel drive system for a motor vehicle and more specifically, to an all wheel drive system including a pair of front halfshaft assemblies, a power take-off unit, a first propshaft assembly, a second propshaft assembly, a means for controlling torque transmission from the front halfshaft assemblies to the rear halfshaft assemblies, a rear differential, a pair of rear halfshaft assemblies and a torque arm mounting system.
There are generally four (4) main types of automotive driveline systems. More specifically, there exists a full-time front wheel drive system, a full-time rear wheel drive system, a part-time four wheel drive system, and an all wheel drive system. Most commonly, the systems are distinguished by the delivery of power to different combinations of drive wheels, i.e., front drive wheels, rear drive wheels or some combination thereof. In addition to delivering power to a particular combination of drive wheels, most drive systems permit the respectively driven wheels to rotate at different speeds. For example, the outside wheels must rotate faster than the inside drive wheels, and the front wheels must normally rotate faster than the rear drive wheels.
Often, the inside and outside drive wheels of a two wheel drive system are connected to a differential mechanism which permits the opposite drive wheels to rotate at different speeds while delivering an approximately equal amounts of drive torque to each. Delivering approximately equal amount of torque to the drive wheels independently of the rotational speed sometimes significantly limits the total amount of torque which can be delivered to the drive wheels when one of the drive wheels loses traction. For example, if either drive wheel does not have sufficient traction to sustain its portion of the drive torque, the amount of drive torque which can be delivered to the other drive wheel is similarly reduced.
In an attempt to overcome this problem, there are certain modifications to differential performance which support unequal distributions of torque between the drive wheels. The unequal distributions of torque are supported by resisting any differential rotation between drive wheels. A limited differential modifies a conventional differential by including a frictional clutch mechanism which resists any relative rotation between the drive wheels. Unequal torque distribution between drive wheels is supported by sacrificing some of the differential capacity to support unequal rotational speeds between the drive wheels during cornering.
Recently, all wheel drive vehicles have been gaining popularity as a way to enhance traction capability. Instead of dividing drive power between only two wheels of a vehicle either in the front or the rear, all wheel drive vehicles divide power between all four wheels. As a result, each wheel is required to support a smaller portion of the total drive torque. However, in addition to delivering power to both the front and rear drive axle, all wheel drives must also permit the two axles to rotate at different speeds. Accordingly, driveshafts to the front and rear axles are often interconnected by a differential mechanism which permits the front and rear drive axles to rotate at different speeds while delivering approximately equal amounts of torque. Part-time four wheel drive systems permit a vehicle operator to selectively connect a second drive axle to the vehicle driveline when adverse traction conditions are encountered. It should be noted however, that if both front and rear axles are permanently interconnected by a differential mechanism, more power is expended by the drivetrain delivering power to two drive axles in comparison to delivering power to only one of the axles. Thus, adequate traction is available for a single pair of drive wheels to support the delivery of drive power and the further division of drive power among more than two wheels is not necessary. Significant power losses and reduced gas mileage sometimes occur as a result of the unnecessary transmission of drive power to additional wheels.
Part-time four wheel drive systems rely on operator judgment to select between two and four wheel drive modes. Driveshafts to the front and rear drive axles are generally coupled together in the four wheel drive mode, thereby preventing the two axles from rotating at different speeds. The drive wheels of one or the other axles tend to skid in response to courses of travel which require the front and rear axles to rotate at different speeds. Accordingly, significant power losses occur in the four wheel drive mode from the tendency of one of the axles to break the vehicle. Power is delivered to either the front or rear axle depending on whichever axle is required to rotate slower to maintain traction. This makes for unpredictable changes in vehicle handling characteristics by switching between effective front or rear wheel drive. Further, part-time four wheel drive vehicles experience the same loss of traction as two wheel drive vehicles until the four wheel drive mode is engaged.
It is also known in the art to provide a conventional differential interconnecting front and rear drive axles with a limited slip differential to further enhance traction capabilities of all wheel drive vehicles. The limited slip differential supports unequal distributions of torque between drive axles, but it also resist relative rotation between the axles. Accordingly, the same power losses occur from permanently driving an additional axle, and drive torque is unpredictably divided between the front and rear axles in response to situations requiring the drive axles to rotate at different speeds.
It is an object of the present invention to provide an all wheel drive system which improves vehicle traction and handling during adverse driving conditions.
It is an object of the present invention to provide an all wheel drive system which incorporates a speed sensing torque transfer device which senses speed difference between the front differential and rear differential and transfers torque upon such speed difference.
It is a further object of the present invention to provide an all wheel drive system which is under normal operating conditions an all wheel drive system which operates with 100% torque to the front wheel drive system and if the front wheels begin to slip the front differential rotates at a higher speed than the rear differential consequently with the power take-off unit and the front propshaft causing the rear propshaft to rotate at a higher speed such that the speed sensing torque transfer device senses the speed difference between the inputs of the rear propshaft and the rear differential and engages whereby the speed sensing torque transfer device transmits torque to the rear differential and wheels until the slip condition at the front wheels is resolved and all wheels are thereby rotating at the same speed.
It is a further object of the present invention to provide a unsymmetrical torque arm for use in conjunction with the speed sensing torque transfer device and rear differential to support the rear differential.
It is a further object of the present invention to provide a preassembled torque arm module which is easily adjustable in the longitudinal (parallel to the propshafts) direction.
It is yet a further object of the present invention to provide a torque arm module including a torque arm which is capable of shielding the speed sensing torque transfer device and rear differential rear propshaft from excessive heat radiated by the exhaust system of the motor vehicle.
It is yet a further object of the present invention to provide a torque arm mount which allows the torque arm assembly to be adjustable in the longitudinal direction.
It is still a further object of the present invention to provide an all wheel drive system which allows the design of the rear halfshaft assemblies and the rear differential to be set at the maximum torque of the speed sensing torque transfer device.
It is yet a further object of the present invention to provide an all wheel drive system having a speed sensing torque transfer device with progressive torque transfer characteristics including a torque limiting feature to protect the rear driveline components.
It is another object of the present invention to provide an all wheel drive system including a speed dependent locking characteristic to minimize sensitivity to tire size variation, tire wear and spare tire or spare wheel usage.
It is a further object of the present invention to provide an all wheel drive system including a speed dependent locking characteristic which is located on the input shaft of the rear differential either internally or in an oil housing or externally mounted.
It is yet another object of the present invention to provide an all wheel drive system including a torque arm mount with an asymmetric configuration which is attached to the front face of the rear differential housing and the motor vehicle undercarriage to reduce the torque reaction load of the rear differential.
It is still a further object of the present invention to provide an all wheel drive system having an asymmetrical torque arm layout which provides improved packaging flexibility and reduced packaging requirements and further acts as a heat shield between the exhaust system and the temperature sensitive driveline components.